Field of the Invention
The present invention generally relates to Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) made on a semiconductor substrate. In particular, the invention relates to making MOSFETs having a single work-function metal for both the N-FET and the P-FET in a replacement metal gate structure.
Description of Related Art
In conjunction with Moore's law, gates of MOSFETs shrink with each technology node. In advanced MOSFETs, the gate may be made from a “replacement metal gate” (RMG) process which requires forming a gate opening in a dielectric layer and filling the gate opening with gate materials as opposed to patterning the gate materials and then surrounding them with a dielectric. Due to the number of work function materials required in current gates, the openings may be difficult to subsequently fill with the bulk material and may result in high gate resistances.
Attempts to create more room for a bulk fill material of a replacement metal gate structure included removing the stack of traditionally used WF metals from the sidewall of the opening. However, there are several drawbacks to work function metal recess: first, it requires several additional photolithography masks, patterning and etch steps; second, the wet etch and reactive ion etch (RIE) processes that are integral to this recess process alter the inherent work functions of the work function metals materials which remain at the bottom of the opening which leads to threshold voltage shifts; third, the repeatability and uniformity of the process is not very good, especially, with varying gate lengths of the transistors within each chip and across the entire wafer; and fourth, with the move to shorter gates (smaller vertical height of the opening) to reduce gate capacitance, the variability and control of work function recess process becomes worse. Thus, there is a need to create more space in the gate opening which provides substantial space for the bulk fill material to achieve low gate resistance while simultaneously providing that the correct work functions are set for N-FETs and P-FETs.
Brief Summary of the Invention
The current invention is a novel structure to both simplify and improve the performance of the replacement metal gate stack for advanced node FETs, and the method of making the same. The current invention allows the correct work functions to be set for N-FETs and P-FETs, while simultaneously allowing substantial space in the gate opening for the bulk fill material which results in lower gate resistance. More specifically, doping of a high-k dielectric film of the gates allows both the N-FET and P-FET gates to share the same, thin, work function metal (single work function metal).
An object of the current invention is a method to form high-k metal gates of a N-FET and a P-FETs by a replacement metal gate process in advanced nodes The replacement metal gate process may be a “high-k first” or a “high-k last” (an embodiment illustrated by the figures) process. The node may be 14 nm and below such that the gate opening may be less than 20 nanometers wide.
A further object of the current invention is that the gates of the N-FET and the P-FET share the same, thin work function metal.
Another object of the current invention is that the gate oxide is doped. The dopants differ in the N-FET gate oxide and the P-FET gate oxide.
Yet a further object of the current invention is to provide a low resistance gate structure by increasing the amount of bulk fill material in the gate opening lined by the work function metal. Despite the gate opening being less than 20 nm, the bulk fill width may be 20-70% of the gate opening width.
An object of the present invention is to form an integrated circuit (IC) including a first gate of an N-FET and a second gate of a P-FET on the substrate wherein the first and second gates have a gate opening width of less than 20 nanonmeters. The IC also includes a work function metal lining both the first gate opening and the second gate opening. Furthermore, a high dielectric material is between the work function metal and the substrate wherein the high dielectric material is doped with an n-dopant in the first gate and is doped with a p-dopant in the second gate. Finally a bulk fill material fills a remainder of gate opening.
Another object of the current invention is a method of forming an N-FET metal gate and a P-FET metal gate sharing the same work function metal, the method comprising. The method provides a dielectric layer over a substrate wherein the dielectric layer has a first opening over an N-FET region of the substrate and a second opening over a P-FET region of the substrate. A high dielectric constant material is formed so as to be in contact with the substrate in the N-FET region and in the P-FET region. A work function metal lines the first and second openings which are subsequently a bulk fill material.
Other characteristics and advantages of the invention will become obvious in combination with the description of accompanying drawings, wherein the same number represents the same or similar parts in all figures.